TANIA AUSTRALIS (Mu Ursae Majoris). Our Ursa Major, the Greater Bear, contains remnants of ancient Arabic constellations, the best-known example the star Alkaid, which refers to the leader of the daughters of a funeral bier. Southwest of the Dipper's bowl lie three obvious pairs of stars that represent the bear's paws, but to the Arabs were the tracks of leaping gazelles. The middle pair is the "second leap," from which comes the name "Tania" (for "second"). In the multi-cultural mix of constellation lore, the northern one received the Arab-Latin name Tania Borealis, the southern Tania Australis. Bayer assigned the three "leaps" ordered Greek letters, Tania Borealis receiving Lambda, Tania Australis Mu. The two make a lovely contrast, Tania Borealis a white class A subgiant, Australis a fairly rare (for naked eye stars) red class M (M0) giant. Tania Australis shines at mid third magnitude (3.05) from a distance of 250 light years (double the distance of Borealis, the two only a line-of-sight coincidence). When we account for infrared radiation from a 3950 Kelvin surface, Tania Australis is found to have a luminosity 850 times that of the Sun, which leads to a radius 62 times solar (0.28 Astronomical Units, three-fourths the size of Mercury's orbit). Having used its core hydrogen, Tania Australis seems to be brightening along the "red giant branch" with a contracting helium core. Before long, the helium will fire up to fuse to carbon, and the star will dim some and stabilize as a class K giant. Tania Australis is an unresolvable binary, the companion (known only from spectroscopic observations) circling the M giant every 230 days at a distance of at least 1.5 Astronomical Units, suggesting a combined mass over 9 times solar, double that expected on the basis of luminosity and temperature. Tania Australis proper (ignoring the companion) is also a rare "hybrid star." Magnetically active stars like the Sun blow a relatively fast but thin wind from their surfaces. Larger giants blow slower, but much thicker winds. Hybrid stars seem to blow both. Though the star is cooler than the dividing line at which stars seem to lose their X-rays, Tania Australis perversely still seems to radiate them. Alas, the southern star of the second leap is not very well understood.
Written by Jim Kaler. Return to STARS.